With an aim of improving wear resistance, sliding characteristic, and protective function, physical vapor deposition methods such as an arc ion plating method and a sputtering method have been known generally as a technique of coating a thin film on the surface of a substrate such as mechanical parts, cutting tools, and sliding parts. In the arc ion plating method, a cathode discharge arc evaporation source is used.
The cathode discharge arc evaporation source generates arc discharge on the surface of a target as a cathode. Thus, a material constituting the target is melted instantaneously and ionized. Then, a thin film is formed by drawing the ionized material to the surface of the substrate as a work. The arc evaporation source has a fast evaporation speed and high ionization rate for the evaporated material constituting the target. Accordingly a dense coating film can be formed by applying a bias to the substrate upon film formation. Accordingly, the arc evaporation source is used industrially for forming a wear resistant coating film, for example, on a cutting tool.
However, when arc discharge is generated between a cathode (target) and an anode, evaporation of the target is generated around an electron emission point (arc spot) on the side of the cathode as a center. In this state, a molten target is released form the vicinity of the arc spot, and is adhered to a work and causes deterioration of the surface roughness.
The amount of the molten target material (macro particle) released from the arc spot tends to be suppressed when the arc spot transfers at a high speed. It has been known that the transfer speed of the arc spot undergoes the effect of a magnetic field applied to the target.
Further, it has been known that target atoms evaporated by arc discharge are electrolytically dissociated, that is, ionized in the arc plasma. There is a problem, for example, that the track of ions from the target to the substrate undergoes the effect of a magnetic field between the target and the substrate.
Further, in a coating film obtained by PVD film formation such as film formation by cathode discharge type arc evaporation source, compressive stress remains in principle. The compressive stress tends to be increased as the thickness of the coating film is larger. Further, when the compressive stress is larger than −2 GPa (compressive stress<−2 GPa), adhesion of the coating film to the tool is lowered and tends to cause peeling. When the thickness of the coating film covering the cutting tool can be increased, the life of the cutting tool can be extended. However, it is difficult to increase the thickness of the coating film due to the reason described above.
For solving such problems, it has been proposed the following attempt of controlling the transfer of the arc spot by applying a magnetic field to the target. For example, Patent literature 1 describes an arc evaporation source having a ring shaped magnetic force generation mechanism (permanent magnet, solenoid coil) disposed to the circumference of a target and applying a vertical magnetic field to the surface of the target. Patent literature 2 describes an ion plating device having a magnetic force generation mechanism (solenoid coil) disposed ahead of the target so as to converge the ionized material constituting the target efficiently in the direction of the substrate. Patent literature 3 describes an evaporation source for an arc ion plating device having a permanent magnet disposed to the central position at the rear surface of the target, a ring shaped magnet disposed at the rear surface of the target so as to surround the permanent magnet and having a polarity different from that of the permanent magnet, and a solenoid coil forming a magnetic field component so as to confine arc discharge and having a diameter substantially equal with that of the ring shaped magnet. Patent literature 4 describes an arc vapor deposition apparatus having a ring shaped magnet disposed to the circumference of a target and a solenoid coil disposed at the rear surface of the target and forming a magnetic field in parallel with the surface of the target by the solenoid coil.
However, according to the magnetic force generation mechanism described in the Patent literature 1, lines of magnetic force from the surface of the target extend to the magnet on the side of the target. Accordingly, most of ions are guided in a direction to the magnet. Further, the lines of magnetic force extending in the direction to the substrate ahead of the target are directed to a direction greatly deviating from the substrate. Therefore, evaporated and ionized material of the target cannot efficiently reach the substrate.
Further, in the technique described in the Patent literature 2, although the lines of magnetic force extend in the direction to the substrate, a large sized solenoid coil has to be placed between the target and the substrate. Accordingly, the distance between the target and the substrate is necessarily made longer and, as a result, the film-forming speed is lowered.
Further, in the arrangement disclosed in the Patent literature 3, arc discharge tends to be generated preferentially at a point where the vertical component of the magnetic field (component in the vertical direction of the magnetic field to the surface of the target) is decreased to 0. Accordingly, even when the solenoid coil is used, the position where the arc discharge is generated is trapped substantially at an intermediate portion between the permanent magnet and the ring shaped magnet and it is difficult to control the position to a further inner circumferential portion. Accordingly, the efficiency of utilizing the target cannot be improved. Further, in the arrangement described in the Patent literature 3, there is no component of the lines of magnetic force extending forwardly from the target. Accordingly, ions emitted from the target are not efficiently converged to the substrate.
Then, the Patent literature 4 only describes an embodiment in which the inner diameter of the solenoid coil is smaller than the diameter of the target. In this case, the lines of magnetic force tend to be diverged from the target to the outside and it is considered that ions cannot be converged efficiently. Further, in the arc vapor deposition apparatus described in the Patent literature 4, discharge of arc plasma is transferred at a high speed in order to obtain a strength necessary for the magnetic field parallel to the surface of the target. Accordingly, in combination with the solenoid coil (or magnetic body yoke), it is necessary to supply a large current to a large-sized solenoid coil to increase the size of the evaporation source which is not preferred industrially.